chapter 3 Concepts of Physics

Force Is a form of energy Causes movement Has direction Has magnitude Gravity is a constant force.

Newton’s Laws of Motion First law of motion: Inertia A body remains at rest or in uniform motion until an outside force acts on it. (continued)

Newton’s Laws of Motion (continued) About acceleration and momentum Second law of motion: Acceleration of an object is directly proportional to the mass of the object being moved. Momentum is the amount of motion a moving object has. Momentum = mass X velocity (continued)

Newton’s Laws of Motion (continued) Third law of motion is about the action-reaction relationship Third law of motion: An object reacts to a force with a force of equal magnitude in the opposite direction.

Center of Gravity Center of gravity (CoG) is the point in a body or an object around which its weight is balanced. Knowing the location of a body or its segments is important. CoG of human body: around the S2 vertebra CoG changes as the body moves. A lower CoG is more stable than a higher CoG. CoG changes with changes in body position (see figure 3.1).

Figure 3.1a

Figure 3.1b

Figure 3.1c

Line of Gravity The line of gravity (LoG) is an imaginary line running vertically through a CoG toward the earth’s center. LoG is perpendicular to the earth’s surface. The line of gravity must fall within the base of support for a body to be stable (see figure 3.2).

Figure 3.2a

Figure 3.2b

Base of Support Includes area of contact with ground Base of support is a two-dimensional area The larger the base of support, the more stability the body has See figure 3.3

Figure 3.3

Figure 3.4

Basic Lever System Lever is a simple machine Contains a rigid bar and fulcrum The body is made up of levers Resistance point is the location of the CoG of the moving segment Resistance arm is the distance between the fulcrum (joint) and resistance point (continued)

Basic Lever System (continued) Force point is where the muscle (force) attaches Force arm is the shortest distance (perpendicular) from the force point to the fulcrum (joint) Lever arm is a moment arm since motion is an arc See figure 3.5

Figure 3.5

Figure 3.6 First-Class Lever

Figure 3.7 Second-Class Lever

Figure 3.8 Third-Class Lever

Figure 3.9 First-Class Lever

Figure 3.10 Changing From a Second- Class to a Third-Class Lever

Figure 3.11 The Angle of Pull Changes as Range of Motion Changes

Figure 3.12 Rotary and Non-Rotary Components of Force

Angle of Pull Maximal force occurs when the angle of pull is 90  See figure 3.13

Figure 3.13

Figure 3.14 Changes in Position Causes Changes in Resistance Arm

Additional Physics Terminology Strength is a muscle’s relative ability to resist or produce a force: –The greater the strength, the greater the force produced –The angle of pull of muscle and resisting force, muscle length, and contraction speed and motion influence muscle strength. (continued)

Additional Physics Terminology (continued) Work = F x d –Work is the product of the amount of force applied and the distance through which it is applied. –Work is measured in foot-pounds or joules (1 J = 1 N-m). Power = Fd/t or w/t –Power is a measure of the work done in a specific amount of time –Measured in ft-lb/s, N-m/s, or j/s (continued)

Additional Physics Terminology (continued) Energy is the capacity to do work. –Different types of energy: Potential energy is the capacity to do work. Kinetic energy is energy the body has because of its motion. Velocity is the rate of change of position (mph, ft/s, or m/s). Acceleration is the rate at which velocity changes (m/s/s or m/s 2 ).

Stress-Related Terminology Elasticity is the ability of an object to resume its former shape after deforming or distorting. Stiffness is the ability of an object to resist deformation when stress is applied to it. Stress is a force that changes the form or shape of a body. Strain is the amount of change in size or shape caused by stress. (continued)

Stress-Related Terminology (continued) Creep occurs when a low-level stress is applied over a period of time long enough to cause deformation. Structural fatigue is the point at which tissue can no longer withstand applied stress and breaks. Friction is the relative resistance between two surfaces. Figure 3.17 demonstrates a use of friction in therapeutic exercise.

Figure 3.15 Physiological Advantage of Muscles

Figure 3.16 Stress-Strain Curve

Figure 3.17a Use of Friction in Therapeutic Exercise

Figure 3.17b Use of Friction in Therapeutic Exercise